A submarine snorkel is a device that allows a submarine to operate submerged while still taking in air from above the surface. It was invented by the Dutch just before World War II and copied by the Germans during the war for use by U-Boats. Its common military name is snort.
Until the advent of nuclear power, submarines were designed to operate on the surface most of the time and submerge only for evasion or for rare daylight attacks. In 1940, at night, a U-boat was safer on the surface than submerged because ASDIC sonar could detect boats underwater but was almost useless against a surface vessel. However, with the continued improvement in methods of radar detection and attack, as the war progressed, the U-boat was forced to spend more and more time underwater running on electric motors that gave speeds of only a few knots and with very limited endurance. A submarine that stayed underwater for more than a few hours also encountered various disposal problems and had to store garbage internally, further fouling boats already infamous for their odors.
The 1940 defeat of the Netherlands by the Wehrmacht, and the capture of O-25 and O-26 was a stroke of luck for the Kriegsmarine. The Dutch had been working on a device that they had named the snuiver (lit: "sniffer"). The Dutch navy had been experimenting as early as 1938 with a simple pipe system on the submarines O-19 and O-20 that enabled them to travel at periscope depth operating on its diesels with almost unlimited underwater range while charging the propulsion batteries.
The Kriegsmarine, at first, gave some consideration to the snorkel as a means to take fresh air into the boats but saw no need to run the diesel engines underwater. In 1943, however, as more U-boats were lost, it was retrofitted to the VIIC and IXC classes and designed into the new XXI and XXIII types.
The first boat to be fitted with a snorkel was U-58 which experimented with the equipment in the Baltic during the summer of 1943. Boats began to use it operationally in early 1944 and by June 1944 about half of the boats stationed in the French bases had snorkels fitted.
On Type VII boats the snorkel folded forward and was stored in a recess on the port side of the hull while on the IX Types the recess was on the starboard side. The XXI and XXIII types both had telescopic masts that rose vertically through the conning tower close to the periscope.
Snorkels created several problems for their users. A U-boat with a snorkel raised was limited to six knots to avoid breaking the tube, and its sound detection gear was deafened by the roaring of the air being sucked down the tube. Most dramatically, snorkels were equipped with automatic valves to prevent seawater from being sucked into the diesels, but when these valves slammed shut the engines would draw air from the boat itself before shutting down, which was extremely painful to the ears of the crew and sometimes even ruptured eardrums. This last problem still exists in later model diesel submarines, but to a limited extent because of high vacuum cut-off sensors that shutdown the engines when the vacuum in the ship reaches a pre-set point.
Modern snorkel induction masts use a fail-safe design consisting of compressed air, controlled by a simple electrical circuit, to hold the "head valve" open against the pull of a powerful spring. Seawater washing over the mast shorts out exposed electrodes on top, breaking the control circuit. This vents the compressed air and allows the head valve to slam shut. When the electrodes are again clear of the water, the circuit is re-energized and the valve reopens.